1. Field of the Invention
The present invention relates to optical, additional films suitably used, for example, in vacuum ultraviolet lithography, particularly, in F2 laser lithography, optical elements in which the optical, additional film or films are added on a substrate, and an optical apparatus using the optical element or elements.
The optical, additional films herein are, for example, antireflection coatings, reflective coatings, or protective coatings, formed on surfaces of optical elements.
2. Related Background Art
Conventional i-line (wavelength 365 nm) and KrF laser (wavelength 248 nm) lithographies, glass materials for optical elements such as lenses, antireflection coating materials, environments, etc. that were applicable to optical systems of optical apparatuses, such as a projection exposure apparatus, illumination apparatus, measuring apparatus, etc., used in lithographies, were a direct extension of the conventional technologies and could be prepared by conventional methods.
As for the lithography in the vacuum ultraviolet region, however, oxygen, water, etc. strongly absorbs light. Thus, it is necessary to keep an atmosphere under vacuum or to replace it with a gas such as nitrogen or helium.
In the ArF laser lithography at the wavelength of 193 nm, conventionally used silica and fluorite were applicable as glass materials and antireflection coating materials and it was also possible to select certain kinds of metal fluorides and oxides.
Silica (SiO2) has been used heretofore as a material for optical elements for an ArF laser. It was because silica (SiO2) has a high transmittance of 90% for an ArF excimer laser light and is stable upon variations in temperature, humidity, and so on. However, the transmittance of silica (SiO2) is substantially lower for the F2 laser light having the wavelength of 157 nm, and it is impossible to use silica as an optical material.
On the other hand, in the F2 laser lithography, only fluorite (CaF2) is applicable as a glass material capable of sufficiently transmitting rays at the wavelength of 157 nm, which hinders the design of optical systems.
Further, the F2 laser lithography also involves a problem concerning optical, additional films, such as the antireflection coatings or the like. In general, the antireflection coatings are formed in a multilayer structure of a combination of a material having a refractive index smaller than that of the substrate with a material having a refractive index higher than that of the substrate, thereby yielding a stronger antireflection effect than that of the antireflection coatings of a monolayer structure.
However, no material with a refractive index higher than that of fluorite has yet been found as a material for antireflection coatings, so that magnesium fluoride (MgF2) or lithium fluoride (LiF) with the refractive index lower than that of fluorite had to be used in monolayer structures in the F2 laser lithography. Therefore, there is a need for a material having a refractive index higher than that of fluorite and being capable of making the optical, additional films (antireflection coatings, reflective coatings, protective coatings) of a multilayer structure in combination with MgF2 or LiF that can be used in F2 laser lithography.
Japanese Patent Application Laid-Open No. 2000-89450 (Application No. 10-272570) suggests reticle materials applicable to the F2 laser lithography. They are crystals of metal fluorides such as MgF2, LiF, and CaF2, which are used so that the crystallographic axis is oriented in a specific direction.